1. Field of the Invention
This invention relates to an image sensor where a thin film transistor (hereinafter, referred to as xe2x80x9cTFTxe2x80x9d) such as a switching element and a photodiode (hereinafter, referred to as xe2x80x9cPDxe2x80x9d) as a photodetector are formed on a single substrate.
2. Description of the Related Art
In an image scanner or facsimile, an image sensor is incorporated for detecting a reflected light from an original surface by which a light is irradiated on an original. An image sensor generally has a configuration in which pixels are disposed in line, each of which consists of a PD made of amorphous silicon (a-Si) as a photodetector and an analogue switch, i.e., a polysilicon TFT. FIG. 3 is a schematic plan view of a pixel in such an image sensor. As shown in the figure, a single TFT is formed to one PD. The PD is connected to a bias line 41 via the TFT, while a gate is connected to a scanning circuit (not shown). Charge generated in each PD is temporally stored in its junction capacitance, and is chronologically read as an electric signal via a signal readout line 42 by driving a switching device by the TFT, at a rate of several hundred kHz to several hundred MHz. In such a TFT driving type of image sensor, reading can be conducted with a single driving IC owing to TFT operation, and thus the number of driving ICs for driving an image sensor may be reduced.
In such an image sensor in which TFTs and PDs are formed in sequence, it is necessary to form the PDs with a thickness of at least one-micron meter for sufficient sensitivity. An insulating film covering the PDs, therefore, must have a thickness of at least 200 nm in the light of its coverage.
Conventionally, such an image sensor has been manufactured by, for example, the procedure shown in FIGS. 5(a) to (k). The cross section in the description below is one taken on line A-Axe2x80x2 in FIG. 3.
Specifically, on a transparent insulating substrate is deposited a polysilicon film 1 with a thickness of 50 to 100 nm by an appropriate technique such as CVD; the film is patterned according to a TFT channel shape by photolithography; and on the film is deposited a gate oxide film 2 with a thickness of 50 to 100 nm (FIG. 5(a)). Alternatively, a polysilicon film may be formed by crystallizing an a-Si by laser annealing, which has been separately formed by CVD.
Then, on the film is deposited a layered structure consisting of a polysilicon or metal film and a silicide film as a gate electrode 3 with a thickness of about 100 to 300 nm, which is then similarly patterned (FIG. 5(b)).
Then, ion doping is conducted for forming source-drain region 4 while phosphorous (P) and boron (B) ions are introduced at given doses for n- and p-types, respectively (FIG. 5(c)).
Subsequently, an SiO2 film as first interlayer film 5 is deposited by CVD with a thickness of 200 to 500 nm, covering the whole surface (FIG. 5(d)).
On first interlayer film 5 is formed lower electrode 6 in a PD, made of a metal such as Cr with a thickness of 100 nm, and it is then patterned into a desired shape (FIG. 5(e)).
On the surface is formed by CVD a p-i-n type of a-Si layer 7 with a thickness of 1 xcexcm consisting of n-, i- and p-layers from the bottom. On the layer are sequentially formed ITO layer 8 as a transparent electrode with a thickness of 100 nm and barrier metal layer 9 such as tungsten silicide with a thickness of 50 to 100 nm (FIG. 5(f)), and then barrier metal layer 9, ITO layer 8 and a-Si layer 7 are patterned by photolithography into the shape of PD 10 (FIG. 5(g)).
On the surface is formed an Si3N4 film as a second interlayer film 11 by CVD with a thickness of about 200 to 500 nm (FIG. 5(h)). As described above, since a-Si layer 7 in PD 10 has a thickness of about 1 xcexcm, it is necessary to form the insulating film covering the PD with a thickness of 200 nm in the light of its coverage.
Then, contact holes 12 are formed, reaching source-drain region 4 for the TFT, gate electrode 3, lower electrode 6 in the PD and barrier film 9, the upper layer in the PD (FIG. 5(i)). Subsequently, a metal 13 such as Al is deposited to a thickness of 500 to 1000 nm, which is then etched into a desired interconnection shape (FIG. 5(j)). Finally, on the surface is deposited an Si3N4 film or organic film such as polyimide as a passivation film 14 with a thickness of 1 xcexcm or less, to form an image sensor as shown in FIG. 5(k).
When forming the contact holes 12 in the step shown in FIG. 5(i), the second interlayer film 11 and the first interlayer 5 are sequentially etched. It is necessary to strictly control etching conditions for forming such deep contact holes. Furthermore, the base layer is inevitably damaged because it is extremely difficult to control an etching endpoint. In particular, during forming a contact hole to a polysilicon layer such as the source-drain region 4, etching damage to the polysilicon may significantly deteriorate device properties. During removing the first interlayer film 5 on the TFT, the PD may be over-etched, which may affect metal films such as the lower electrode 6 in the PD and the barrier film 9 to some extent.
Thus, the interconnections in the TFT and the PD may be separately formed for avoiding forming such a deep contact hole. FIGS. 6 ((a) to (g)) is a process cross-section showing the procedure.
First, a TFT is formed and then the first interlayer film 5 is deposited as described above (FIG. 6(a)). Then, the interconnection layer 13a a for the TFT is formed (FIG. 6(b)), and on the interconnection layer 13a is formed the third interlayer film 15 such as a silicon oxide film with a thickness of about 200 to 500 nm.
Then, the lower electrode 6 (FIG. 6(d)) for the PD and the PD 10 (FIG. 6(e)) are formed as described above. Subsequently, the second interlayer film 11 is formed, covering the whole surface as described above (FIG. 6(f)), and finally the interconnection layer 13b for the PD is formed (FIG. 6(g)).
Thus, interconnection layers for the TFT and the PD may be separately formed for avoiding damage to the base layer during forming contact holes. It, however, requires two interconnection-forming steps, leading to a complicated process.
An objective of this invention is to provide an uncomplicated process for manufacturing an image sensor where damage to a base layer is minimized during forming a contact hole, as well as an image sensor structure prepared thereby.
This invention which can solve the above problems provides an image sensor comprising a thin film transistor on a transparent substrate, the first interlayer film covering the thin film transistor, a photodiode as a photodetector on the first interlayer film and the second interlayer film on the photodiode and the first interlayer film, where the first and the second interlayer films are made of different materials and at least a contact hole to the element consisting of polysilicon in the thin film transistor is formed after removing the second interlayer film around the area where contact holes are to be formed.
In particular, it is preferable that the contact hole formed by removing the second interlayer film is for a source-drain region or the source-drain region and a gate electrode, and the second interlayer film is removed over the whole surface of the thin film transistor or over the whole surface of the thin film transistor and an area sufficient to expose a part of the lower electrode in the photodiode. It is more preferable that the first and the second interlayer films are a silicon oxide film and a silicon nitride film, respectively.
This invention also provides a process for manufacturing an image sensor, comprising the steps of (1) forming a thin film transistor on a transparent substrate, (2) forming the first interlayer film covering the thin film transistor, (3) forming a photodiode as a photodetector on the first interlayer film, (4) forming the second interlayer film made of a different material from that for the first interlayer film, on the photodiode and the first interlayer film, (5) removing the second interlayer film at least around the area where are to be formed contact holes to the elements consisting of polysilicon in the thin film transistor, (6) forming contact holes in the first interlayer film exposed in the area where the contact holes are to be formed, after removing the second interlayer film and (7) forming interconnection layers for the thin film transistor and the photodiode.
In particular, a preferable aspect of this invention is the process described above wherein a contact hole for connecting an interconnection layer to the second interlayer film is formed while forming a contact hole in the first interlayer film exposed in the area where the second interlayer film has been removed for forming the contact hole, and wherein the first and the second interlayer films are adjusted in their thickness according to the etching rates for the first and the second interlayer films by the etchant used during forming the contact holes to the first and the second interlayer films.
Another preferable aspect of this invention is the process described above wherein a contact hole for connecting an interconnection layer to the second interlayer film while removing the second interlayer film around the area where are to be formed contact holes to the elements consisting of polysilicon in the thin film transistor.
This invention can provide an image sensor structure with reduced damage to the base polysilicon without complicating the manufacturing process.